(a) Field of the Invention
The present invention relates to an electrostatic imaging device having a brush charger and, more particularly, to an electrostatic imaging device for transcribing a toner pattern on the surface of an electrostatic latent image carrier onto a recording sheet such as a recording paper, OHP (overhead projector) sheet or postcard.
(b) Description of the Related Art
Electrophotography processes is generally used in an electrostatic imaging device (often referred to simply as an "imaging device" hereinafter) such as a copying machine, facsimile, electrophotography or electrostatic printers. In the imaging device, after a photoreceptor drum (or electrostatic latent image carrier) is electrified to form an electrostatic latent image, electrified or charged toner is supplied to the surface of the photoreceptor drum As a result, the electrified toner is adhered to the photoreceptor drum having a latent image pattern thereon, to form a toner pattern for development. Recording sheet is then supplied to the surface of the photoreceptor drum in synchrony with the rotation of the photoreceptor drum while applying a bias voltage, which has an opposite polarity with respect to the toner voltage, from the rear side of the recording sheet by using a transcribing roller, thereby transcribing the toner pattern formed on the photoreceptor drum to the recording sheet. Subsequently, the toner pattern on the recording sheet is fixed to the recording sheet by using a fixing unit.
A corona charging device, which generates a corona discharge from a wire electrode, is generally used for electrifying the photoreceptor drum. A a power source is used to apply a high voltage of several kilovolts to the wire electrode for generating the corona discharge. The power source of such a high voltage is highly expensive to thereby raising the fabrication cost of the imaging device. In addition, ozone generated by the corona discharge produces an undesirable influence on the human body and the natural environment. Further, there arises a problem in that the photoreceptor drum and other units disposed around the photoreceptor drum are damaged by the corona discharge.
As an alternative for the corona charging device, a contact-type electrifying device is known which electrifies the photoreceptor drum by using a brush charger. The brush charger is classified into two categories including a planar brush of a fixed type and a roller brush of a rotational type. The fixed-type brush charger generally comprises an insulator substrate, a conductive layer fixed on the insulator substrate and applied with a bias voltage, and low-resistive brush fibers extending from the conductive layer.
The brush charger as mentioned above can solve the problems involved in the corona charging device. However, it involves another problem as described below. Specifically, in the electrostatic imaging device, the drum surface is generally subjected to abrasion by the friction between the photoreceptor drum and the developing roller, which sometime produces dispersed particles or dust charged in opposite polarity with respect to the photoreceptor drum. The friction may also generate toner particles electrified in an opposite polarity in addition to toner particles electrified in the normal polarity. These dispersed particles or toner particles charged in opposite polarity are generally firmly attached to the surface of the photoreceptor drum and are difficult to remove by using a cleaning unit.
In addition, the rotating photoreceptor drum allows these undesirable particles on the photoreceptor drum to be introduced between the fibers of the brush charger and attached firmly thereto, or allows the undesirable particles to be adhered to a contact-type transcribing member. In these cases, the photoreceptor drum is subjected to an insufficient electrified state for printing or image forming due to the undesirable particles, whereas the transcribing roller makes fog on the rear surface of the recording sheet due to the undesirable particles.
Patent Publication No. JP-B-63 (1988)-43750 proposes an electrostatic imaging device wherein the undesirable particles attached to the brush charger are efficiently removed by applying a voltage having a DC voltage component in the opposite polarity with respect to the polarity of the photoreceptor drum to the brush charger during a non-printing period (period other than an image forming period) of the imaging device, whereas applying thereto a voltage having a DC voltage component in the same polarity with respect to the photoreceptor drum during a printing period (image forming period) of the imaging device.
In the proposed imaging device, the surface of the photoreceptor drum is subjected to electrification in the opposite polarity with respect to the polarity of the drum surface during the period other than the image forming period, which provides the advantage of easier attachment of the toner to the drum surface during the period other than the image forming period of the imaging device. The easier attachment of the toner allows the undesirable particles to be removed from the brush charger for cleaning. However, it involves another problem wherein the charged toner is more firmly attached to the drum surface when the portion of the drum surface charged in opposite polarity is in contact with the toner supply member, or developing roller, after some rotation.